For example, in a suspension mechanism of a vehicle, in order to rotatably and vibratably connect two portions (for example, a stabilizer and a shock absorber, or a stabilizer and a suspension arm) which can move relatively, a ball joint may be used.
This ball joint is equipped with a ball stud, a ball seat, a housing, a support bar, and a dust cover. The ball stud has a stud portion and a ball portion. The ball seat has a bearing function and the ball portion is press-fitted therein. The housing holds the ball seat therein. The support bar supports the housing. The dust cover prevents infiltration of foreign matter which may inhibit sliding and vibration of the ball portion of the ball stud in the ball seat.
In a production method for this ball joint, a housing and a support bar were separately formed, and the support bar, which was made of steel (for example, S200C), was connected to the housing, which was produced by press forming or the like, by resistance welding or carbon dioxide welding.
When the support bar and the like are made of a steel material, the support bar is the heaviest of the components (the ball stud, the dust cover, the support bar, and the ball seat) of one ball joint. For example, the weight of the support bar having an axial direction length of 325 mm is 260 grams, the total weight of the components other than the support bar is 130 grams, and the weight of support bar is two times as heavy as those of the components other than the support bar. That is, the weight (130 grams) of the support bar is two thirds of the weight (390 grams) of the ball joint. Due to this, the weight of the ball joint is heavier, and it is therefore difficult for the ball joint to contribute to the weight reduction of the vehicle.
When the housing and the support bar are separately formed, the support bar is connected to the housing by resistance welding or the like. Thus, when endurance tests were performed in which a tensile load or a compressive load was applied on the support bar in an axial direction thereof, it was confirmed that breakage occurs at a welded portion before a predetermined number of times of conducting the endurance test. Due to this, in a housing and a support bar which are integrally formed, a strength of a connecting portion may be low, and reliability of a ball joint may be insufficient.
For example, Japanese Unexamined Patent Application Publication No. 2004-316771 discloses a production method for ball joints in which a housing and a support bar which are integrally formed by using aluminum or the like. In this production method, first, a resin liner (ball seat) is formed at a ball portion of a ball stud by injection molding. Next, the ball portion and the resin liner are inserted into a die for forming a housing and a support bar as a core. A housing, which covers the resin liner, and a support bar, which supports the housing, are formed by die casting using a zinc alloy or an aluminum alloy. Next, a stud portion is connected to the ball portion.
However, in the technique proposed above, in the die casting, a melted aluminum alloy is supplied to the die using the ball portion of the ball stud and the ball seat as the core, so that an outer surface of the ball seat made of a resin is deteriorated. Due to this, a contact portion of the ball seat which contacts the ball portion is maintained in good condition, but resin thickness thereof has a subtle unevenness. Thus, sliding and vibration of the ball stud may be unstable. That is, when a tensile load or a compressive load is applied on the support bar in an axial direction of the ball stud and in a radial direction of the ball portion, deflection amount of the ball seat may be uneven. Due to this, sliding and vibration of the ball stud may be irregular. As a result, the performance of the ball joint cannot be sufficient, and the ball joint may not be sufficiently reliable.
Since the ball portion of the ball stud and the ball seat are used as the core in the die casting, when the stud portion is connected to the ball portion, die casting cannot be performed for the formation of the housing and the support bar. Due to this, after the die casting for the formation of the housing and the support bar, the stud portion was connected to the ball portion by resistance welding or the like, and the ball stud was thereby formed. Thus, strength of a connecting portion of the ball portion and the stud portion may be low, and reliability of the ball joint may be insufficient.